What it means
The point of a broad-based index is coverage. Rather than tracking a handful of names or a single sector, it samples enough of the market that no one company or industry can dominate the result.
That coverage matters because investors, trustees and boards use these indices as benchmarks. When a fund manager says the portfolio returned 9% last year, the first question is what the broad market did over the same period, and a broad-based index is the usual answer.
Most broad-based indices are weighted by market capitalisation, meaning each company counts in proportion to its total share value. A few use equal weighting, where every constituent counts the same, and a small number weight by share price instead.
The weighting method changes the story a broad index tells, so it is worth checking which one you are looking at. There is also a regulatory sense of the phrase.
Under United States securities and options rules, an index is treated as broad-based when it is diversified enough that trading in any single stock cannot move it much, and that classification affects how index options are taxed and margined. The main nuance is that broad does not always mean neutral.
A capitalisation-weighted index of 500 companies can still end up with a third of its value sitting in a dozen large technology names, so a manager who claims to be diversified because they track a broad index may be far more concentrated than they think.
In practice
Real-world examples.
Example
The trustees of a $180,000,000 charity endowment set their equity benchmark as a broad-based index covering the whole domestic market. When their active manager returned 7.2% against the index level rise of 9.1%, the trustees had a specific number to discuss at the next meeting rather than a vague sense that something was wrong.
Example
A software company's remuneration committee ties a third of executive long-term awards to shareholder return measured against a broad-based index rather than a technology sector index. The reasoning was simple: a narrow sector index would reward management for being in a hot industry rather than for running the business well.
Example
A family-owned manufacturer reviewing its staff retirement plan found that 4 of the 6 fund choices were single-sector funds. It added a low-cost fund tracking a broad-based index and made it the default, so employees who never made an active choice ended up diversified instead of concentrated.
Formula
Calculation
A capitalisation-weighted broad-based index is calculated as:
Index level = Total market capitalisation of all constituents / Divisor
Take an index covering 3,000 companies with a combined market capitalisation of $12,000,000,000,000 on the day it launches at a base level of 1,000. The divisor is set so that the base level comes out right: $12,000,000,000,000 / 1,000 = $12,000,000,000.
A year later the combined market capitalisation of those same 3,000 companies is $12,600,000,000,000. The index level is $12,600,000,000,000 / $12,000,000,000 = 1,050, a rise of 50 points or 5%.
Individual weights fall out of the same figures. A constituent worth $600,000,000,000 represents $600,000,000,000 / $12,000,000,000,000 = 5% of the index, so a 10% fall in that one share, with everything else unchanged, would pull the index down by 0.5%.Case study
Seen in the real world.
Northwind Trellis Asset Management is an illustrative firm invented to show the point. It marketed a fund called the Whole Market Portfolio, promising clients exposure to the entire listed market through a single holding, and for several years the pitch worked well.
During a routine review, the investment committee itemised the fund's largest positions and found that the top 10 holdings accounted for 31% of its value. Nothing had gone wrong; the fund was doing exactly what a capitalisation-weighted broad-based index does, which is to let the biggest companies grow into the biggest weights. Clients who thought they owned 2,500 companies in roughly similar amounts were, in practice, making a large bet on a small group of them.
Northwind Trellis did not abandon the index. It kept the broad-based fund as the core, added a smaller equal-weighted sleeve for clients who wanted the concentration diluted, and rewrote its factsheet to show the top 10 weights on the front page. The fictional lesson is that a broad index is an honest description of the market, not a guarantee of even exposure.
Watch out
Common mistakes.
- Assuming a broad-based index gives equal exposure to every company in it, when most of these indices are capitalisation-weighted and the largest constituents dominate performance.
- Comparing a specialist fund against a broad-based index and concluding the manager is skilful, when the fund's sector simply had a good year relative to the market.
- Treating the number of constituents as the measure of breadth, when a 3,000-stock index concentrated in one sector is narrower in substance than a 500-stock index spread across the economy.
Questions
People also ask.
How many companies does an index need to be broad-based?
There is no fixed threshold, but indices with several hundred or more constituents spread across most industries are generally described this way, and regulators add their own diversification tests for options purposes.
Is a broad-based index the same as the whole market?
Not quite, since most exclude very small companies, foreign listings and firms with too little freely traded stock, so they are a large representative sample rather than a complete census.
Can you invest directly in a broad-based index?
No, an index is only a calculation, so you invest through an index fund or exchange traded fund that holds the constituents and tracks the index level.
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